1. Field of the Invention
The present invention relates generally to gasification of carbonaceous materials to useful fuel gases and other products. More particularly, the invention pertains to methods and apparatus for generating a highly reactive gasifying agent and uses thereof in thermal gasification processes.
2. State of the Art
Thermal gasification using superheated steam is a well-known art. In a typical thermal gasification process, a carbonaceous material such as coal, cellulosic waste, or other carbon-containing material is reacted with steam or a hot gas at temperatures greater than about 1400° F. (760° C.), to produce a combustible fuel gas largely composed of carbon monoxide (CO) and hydrogen (H2). Also, carbon dioxide (CO2) and water vapor (H2O) are generally present in substantial quantities. Methanation, which increases exponentially with pressure and decreases with increasing reactor temperature, also occurs to produce hydrocarbons e.g. methane. Small amounts of other gases such as ethane and ethylene may also be produced. The gasification conditions are controlled to yield a product gas for use as a fuel or as a feedstock for making other hydrocarbon fuels, ammonia, methanol, hydrazine, and other chemical species.
The well-known chemical reactions which occur in thermal gasification of carbonaceous materials include the following endothermic equations:C+H2OH2+CO−56,520 BTU/lb-mol carbon  (1)C+2H2O2H2+CO2−38,830 BTU/lb-mol carbon  (2)
The actual composition of the product gas is influenced by many factors, including the quantities and composition of incoming feed materials, gasification temperature, pressure, and reactor residence time.
Thus, starting with a set of chemical component input and gasification conditions, the actual composition of the product gas is calculated by consideration of reaction rates, chemical equilibria, mass balances, and thermal balances. In some systems, catalysts are utilized to change the reaction rates and shift the composition of the product gas, i.e. syngas.
A major concern in developing workable processes for gasifying materials such as coal and biosolids is the high thermal energy requirement for driving the endothermic reactions.
In most gasification processes, substantial heat must be provided to satisfy the highly endothermic chemical reactions. This heat is typically provided by either (a) partially combusting the incoming carbonaceous material, (b) exothermically reacting a material such as calcined lime with carbon dioxide, and/or (c) by providing heat from an outside source, e.g. hot char circulation, addition of excess steam, etc.
In some gasification systems, mixtures of air and steam are used as the gasifying agent, and some or all of the required heat is provided by oxidation of a portion of the carbonaceous feed material within the gasification reactor. In such systems, heating of the inert nitrogen gas in the air wastes energy, and the produced gas will contain a substantial fraction of free nitrogen, resulting in a low heating value.
Gasification with a mixture of steam and pure or enhanced oxygen gas has been promoted, but full development has been hindered because (a) a large portion of the carbonaceous material is combusted to non-fuels (CO2 and water), and (b) the resulting product gas contains a low ratio of hydrogen gas to the total of carbon dioxide and carbon monoxide. The primary industrial need is for gases with higher H2:CO ratios, because hydrogen is used for hydrogenation, a common chemical engineering practice, and shows great potential for use in fuel cells. Hydrogen has a high value in the chemical industries, and its oxidation byproduct is water, a non-pollutant.
Steam-only gasification has been investigated and used commercially since about 1950-1960. It is usually desirable to maintain a steam:carbon ratio which is close to a value at which the carbon is fully reacted by reactions (1) and (2) above, with minimal excess steam. More particularly, the conversion of carbon to CO should be maximized, as in reaction (1). Thus, an extraneous heat source is usually provided to supply the necessary heating requirements. The product gas typically has a higher H2:CO ratio than when gasifying with a mixture of steam and air or oxygen. However, because of the limited heat in the steam, the problems associated with steam-only gasification include low achievable reaction temperatures i.e. typically less than about 1500° F. (815° C.), where long residence times and high energy consumption prevail. To operate at higher temperatures, complex heat transfer systems are utilized in order to avoid intermingling of combustion gases with the gasification products. Such systems entail high capital and operating costs, and are generally considered to be uneconomic.
In U.S. Pat. No. 4,004,896 of Soo, it is proposed to operate a thermal gasification system with a large quantity of excess steam, i.e. 2-10 times that required for full gasification of the carbon. In Soo, the thermal requirements of gasification are provided by copious quantities of steam. However, the quantities of H2 and CO produced per pound of steam are low.
The use of high temperature superheated steam for gasification processes has been proposed. In a system configuration described in Emerging Technology Bulletin No. EPA/540/F-93/XXX entitled SPOUTED BED REACTOR, dated August 1993, by the U.S. Environmental Protection Agency, streams of methane and pure oxygen are fed to a burner, with the hot flame injected into a stream of low temperature steam which is passed into a primary gasification reactor. The gasification temperature is partially maintained by oxidation of portions of the feed material and gases leaving the reactor. The injected steam supplies only a portion of the heat required to maintain the low gasification temperature.
U.S. Pat. No. 3,959,401 of Albright et al. describes an apparatus for cracking gaseous and liquid hydrocarbon feedstocks to other chemicals, using a hot gas. It is stated that a hot gas temperature up to 3000° C. (5432° F.) may be used. The source of the hot gas and its composition is not indicated. Furthermore, the sole purpose of the hot gas is to supply heat for the endothermic cracking reactions. The hot gas does not react to become part of the product. The purpose of the apparatus is cracking, and gasification of carbonaceous materials to CO and H2 is not in view.
In U.S. Pat. No. 4,013,428 of Babbitt, an oxygen blown system for gasifying powdered coal is described. A fuel is pre-burned with oxygen to form a mixture of steam and CO2 to which a small amount of water is added. The combustion temperature is indicated to be about 4722° F., and the gas is contacted with the powdered coal to produce a product gas. Each of fuel, oxygen and steam is separately introduced into the pre-burner.
Babbitt also describes a process in which the pre-burner is fed separate streams of fuel, air and steam, creating a gasifying agent containing CO2, steam and inert nitrogen at a temperature of about 3770° F. The presence of nitrogen is detrimental to energy efficiency and results in a product gas of lower heating value.
In U.S. Pat. No. 2,672,410 to Mattox and U.S. Pat. No. 2,671,723 to Jahnig et al., a mixture of oxygen and steam is introduced into a gasifier vessel. The mixture is passed through a porous distribution plate into a bed of gasifier feed material, a portion of which is combusted by the oxygen to generate heat for endothermic gasification.
In U.S. Pat. Nos. 2,631,921 and 2,681,273 to Odell, a mixture of steam and oxygen is passed through a porous distribution plate into a stationary bed of gasifier feed material, or a bed or catalyst or packing solids with high surface area. The batch process is started by initially combusting a fuel below the bed to ignite the bed, then blasting with air until the bed reaches and maintains gasification temperatures.
In U.S. Statutory Invention Registration (SIR) number H1325 to Doering et al., a coal gasification process is described in which oxygen and steam are added to a stream of coal and recycled flyash. The mixture is introduced into a gasifier reactor, where partial combustion occurs.
U.S. Pat. No. 6,048,508 to Dummersdorf et al. discloses a gasification process in which a portion of the synthesis gas from a secondary reformer is cooled and passed through a multistage gas separation plant to separate CO from the other components. The CO is used for other processes, while the remaining other components are returned to the gas stream downstream from where the gas was drawn off, to be treated in a CO conversion stage with the rest of the raw synthesis gas.